Topic: CB750 Timing Cant be Aligned, Not Enough Rotation

[mkoski]

First off, I'm re-posting this question from my project log in the projects forum to reach a larger audience, I'm sorry if this is against the rules or something like that!

Facts:

CB750K, 1977, stock points plate, bike runs fine besides a bit of a rough idle, a bit rich and some noise under the valve cover. Points gapped to 15 thou with feeler gauge. Points being set in accordance to http://www.hondachopper.com/garage/carb_info/timing/Static_and_Strobe_Timing.pdf. I have tried everything I can think of to align my ignition timing and now I turn to you guys!

Problem:

When setting ignition timing my 1-4 F mark is to the left of the timing mark. I have rotated my points plate all the way possible clockwise to try and bring it into alignment but its just not possible with the slots allocated for the set screws. The point gap is to spec and I just cant see anything else that I can possibly do to fix this. Does anyone have any ideas or advice?

As always, thanks for the help,

MJK

[flybox1]

There is no "need" to switch away from points, And the Dyna will just draw more power, without much , if any, spark improvement.   However, if you are unable to make the points system work, it could get you driving.

The points are a simple and basic electrical switch.  When closed they draw current through them, charging the coil.  Open, allows the coils to spark the plugs.  There are only two electrical connections.  One side connects to the engine case, the other connects to the coil.  Pay attention to the wire terminals on the points so they aren't allowed to contact the engine case, and be certain the insulation collar on the post is centered.

Once the connections are confirmed, rotate the crank so that the points are at their most open position.  Ignore the timing marks for now.  Set the mechanical gap to book spec.  If you apply power now, an attached light will shine bright as the lamp filament provides an electrical path around the point contacts.  Now rotate the crank until the light goes out.  This is where the 1.4 F mark should align.  The main plate screws are loosened just enough to allow the plate to rotate.  If the main plate is undersize, the spring pressure from the points can move the main plate in the engine bosses and this can change the gap.  The crank's cam position can alternate which spring is dominant as the crank is turned...which changes the gap setting you just made, because the plate can move with loosened screws.  I like to shim the plate, so it can't move laterally in the engine bosses.  The shims I use are feeler gauge "fingers".  Now, when you rotate the plate, the 1.4 point gap doesn't change, and you can rotate the plate to a position where the light will come on as the F mark aligns with marker.  Lock all three screws of the main plate down tight and do a final check of both max gap on the highest point of the cam on the crank, and that the light comes on when the marks align. You can remove the shim now.  Next move on to the 2.3 point set.  Again rotate the points cam to make the points open their widest and adjust the gap.   Then rotate the crank for 2.3 F mark alignment.  The 2.3 points have their own sub plate, which is moved on top of the main plate, to get the light to illuminate as the marks align.  In most cases, you are now done with point timing.  If ever you loosen the 1.4 plate screws for any reason, you will need the shim for the plate so the plate will hold lateral position, particularly if you attempt to use a dynamic timing light. I've done both, and no longer use the Dynamic to set basic timing, only to verify that the advance is working properly.

If you "run out of range" on either point set, you'll need to pick another of the three engine bosses to place the shims, and start over from the beginning with the 1.4 points.  I have yet to develop a rule about which mount boss to shim.  I think I used the lower left, last time I did this.   Now I just insert shims where ever is the largest before loosening the three screw for the 1.4 plate, 'cause that's where it was needed last time I did the adjustment. No need to do the same work twice.

There is a FAQ entry about shimming the points plate.

[mkoski]

Flybox, you are the best, you and Calji737 have been fantastic.

I'll try the shimming and perhaps put more focus on static timing instead of 100% on dynamic. This is just the kind of thing I was looking for. Will report back this weekend on if this solved my issues.

Thanks,

MJK

[flybox1]

haha.  Not me, man.  TwoTired to the rescue!

[scottly]

Are the points Diachi brand, or the preferred TEC brand?

[evanphi]

You can also get the static timing close, and then do a dynamic timing "live" by adjusting the points gap (there is a range, remember!). I did the shimming and everything... still couldn't get them set. Opening the gap advances the timing. My gap is set to 0.011" now... no ill effects. FWIW I have Diiachi condensers, but they are working just fine. Check my build thread for a bit of info (last page).

[calj737]

Are the points Diachi brand, or the preferred TEC brand?

Thats what I asked in his Project thread  Sounds like Daichi strikes again!

[mkoski]

Nope, the points are TEC!

Good news is is that I got my timing set bang-on over the weekend. The issue was a combination of me reducing the gap and shimming the plate. It now fires just a touch early but the advance is perfectly centered on the mark. Noticeable improvement.

Thanks to everyone that helped me out on this, thats one more problems behind me.

MJK